Many applications involve the use of a photon source (or light source) to produce electromagnetic radiation at specific wavelengths or wavelength ranges, e.g., in the ultraviolet (UV), visible, or infrared (IR) range. For example, an analytical instrument may utilize photons to perform optical-based measurements on a sample (e.g., absorbance, reflectance, transmittance, luminescence, light scattering, fluorescence, phosphorescence, etc.), or to form analyte ions from a sample by photo-ionization in preparation for spectrometric analysis (e.g., mass spectrometry or MS, ion mobility spectrometry or IMS).
A spectrometry system in general includes an ionization apparatus (or ion source) for ionizing components of a sample of interest, an analyzer for separating the ions based on a discriminating attribute, an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce user-interpretable spectral information. The spectral information may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized. In an MS system, the analyzer is a mass analyzer that separates the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”). Depending on design, the mass analyzer may separate ions by utilizing electric and/or magnetic fields, or time-of-flight tubes. In an IMS system, the analyzer is a drift cell that separates ions based on their different collision cross-sections. Ions are pulled through the drift cell by a DC voltage gradient in the presence of a drift gas. Ions of differing cross-sectional areas have differing mobilities through the gas environment. An IMS may be coupled with an MS to provide unique two-dimensional information about an analyte under investigation. Additionally, in certain “hyphenated” or “hybrid” systems the sample supplied to the ionization apparatus may first be subjected to a form of analytical separation. For example, in a liquid chromatography-mass spectrometry (LC-MS) system or a gas chromatography-mass spectrometry (GC-MS) system, the output of the LC or GC column may be transferred into the ionization apparatus through appropriate interface hardware.
An electron ionization (EI) source is commonly deployed as the ionization apparatus in MS. The EI source utilizes a high-energy (70 eV) beam of electrons to break analyte molecules into a range of ionized fragments, which are then utilized to produce a mass spectrum. While the EI technique is well suited for targeted analysis in which a measured mass spectrum is compared against spectral libraries, it is poorly suited to analysis of unknown compounds. For analysis of unknown compounds it is desirable to produce either molecular ions (in which the analyte is left intact with only a single electron removed) or a limited number of high-mass fragments. This can be achieved by “soft ionization” in which the energy of the particles used for analyte ionization is high enough to ionize but too low to cause significant fragmentation (e.g., 8-12 eV). While low-energy electron beams may be utilized to achieve this, their current is space-charge limited and their energy spectrum relatively wide (several eV). Soft ionization may also be implemented by chemical ionization (CI), which utilizes an electron beam or corona discharge in conjunction with an added reagent compound such as methane, adding cost and complexity to the instrument.
An alternative to EI and CI is photo-ionization (PI) in which ionization results from irradiation of the sample by ultraviolet (UV) photons. Ultraviolet PI is becoming recognized for its ability to ionize many chemical species, both polar and non-polar, with reduced fragmentation and with retention of high sensitivity and dynamic range as compared to other ionization techniques. With the appropriate choice of photon wavelength (energy), efficient analyte ionization and low levels of undesired ionization of non-analytical components such as solvents can be achieved simultaneously. The UV photons are typically produced by a plasma source. As there is no space charge limitation for photonic radiation, the flux of UV photons is limited only by how strong of a flux can be produced by a plasma source. Additionally, UV emission spectra are typically extremely narrow (e.g., much less than 1 eV).
To confine and isolate the UV-emitting plasma away from the photo-ionization region, the plasma source often employs a window that is transparent to UV, which may have a composition such as magnesium fluoride (MgF2), calcium fluoride (CaF2), or lithium fluoride (LiF). However, materials with good transmission in this spectral range have a tendency to degrade as a result of UV transmission and thus have limited operating lifetimes. Additionally there are some UV energies for which no suitably transparent materials exist. Moreover, windows of any type present solid surfaces and thus are prone to coating by analytes or contaminants, further impairing UV transmission and requiring cleaning. To avoid these problems, windowless plasma UV sources may be employed. Lacking a window, plasma particles are free to stream out into the ionization region. This plasma plume can have a negative effect on the ion source operation. High energy species (including ions, electrons, and excited metastable atoms) can cause unwanted fragmentation, and significant amounts of plasma ions can appear in the mass spectra. The UV light emitted by such devices is not collimated, and transmissive optics for light at these wavelengths exhibit the same limitations as those described for windows. Consequently, it is advantageous to locate the source of UV photons as close as possible to analyte molecules. This proximity however also increases the undesirable plasma flux that enters the photo-ionization region.
The challenge then is to deliver UV to the ionization region while simultaneously limiting both plasma interaction with analyte molecules and plasma ion contamination of the mass spectrum. One known approach is to place electrostatic optics between the plasma source and the ionization region. The optics are then biased such that the plasma ions and electrons are deflected away from the ionization region. This approach is limited by the propensity for DC plasma discharges to form outside of the confining structure of the plasma source when biased surfaces are nearby, exacerbating the issue of plasma interaction/contamination. Also, plasmas exhibit a property in which electric fields are shielded, which limits the effectiveness of electrostatic optics. These optics can also shadow UV emission, reducing the amount of UV photons delivered into the ionization region.
Therefore, there is a need for photon sources and PI sources capable of producing high photon flux levels while minimizing or eliminating the problems noted above.